The Global System for Mobile communication (GSM) is a radio communication system used by public land mobile networks (PLMNs) in many countries. The system is defined by a GSM standard that ensures uniformity and interoperability so that a user can access a GSM-compliant system anywhere in the world with minimal equipment compatibility problems. In addition to details such as modulation, frame formats, etc., the GSM standard specifies other activities that can be performed in the system. Many activities are associated with each particular subscriber, including call-related events such as call set-up and call termination. Other types of subscriber activities include invocation of call-related and call-independent supplementary services such as call hold, call waiting, call transfer, and call forwarding. Based on roaming agreements between different mobile network operators, a mobile subscriber belonging to a specific (home) PLMN (HPLMN) can use subscribed services and facilities while visiting (roaming in) other PLMNs (VPLMNs).
A network architecture typical of GSM systems is illustrated by FIG. 1, which shows an HPLMN 101 and VPLMNs 103, 105. In the HPLMN 101, a home location register (HLR) 115 stores data relating to subscribers to that PLMN, including, for example, current location of the subscriber equipment, directory number (MSISDN), radio number plan identification (e.g., international mobile subscriber identity (IMSD)), supplementary service profiles and teleservice profiles. A mobile switching center (MSC) and visitor location register (VLR) 120 manage connections and data associated with subscribers who are currently situated within their area of responsibility, e.g., those subscribers to other PLMNs, such as the HPLMN 101, who are roaming in their service area in the VPLMN 103 and their own subscribers belonging to the network operator that controls the VPLMN 103. Although actually different nodes, the MSC and VLR are almost always co-located; thus, they are usually referred to as one MSC/VLR.
An authentication center (AUC) 125 works in close association with the HLR 115, providing information for authenticating all communication sessions in order to guard against possible fraud, stolen subscriber cards, and unpaid bills. When a subscriber terminal 130, such as a mobile telephone, that is a subscriber to the HPLMN 101 contacts a base station (BS) 135 and tries to register their presence while visiting PLMN 103, the MSC/VLR 120 requests information from the HLR 115 in order to determine whether the terminal 130 is authorized to use the subscribed services and facilities within VPLMN 103. The information is requested by the HLR 115 and stored therein, and the HLR 115 transmits the information to the MSC/VLR 120 in response to a request from the MSC/VLR 120, e.g., during location updating. The MSC/VLR 120 transmits some of the information to the terminal 130, which determines a response that it sends to the MSC/VLR 120. Based on the response and the information, the MSC/VLR 120 determines whether the terminal 130 is authorized, and the HLR 115 updates its database to indicate that the terminal 130 is located in the area served by the MSC/VLR 120.
When the terminal 130 moves from the VPLMN 103 to the VPLMN 105, the terminal 130 recognizes the change and sends a location updating message to the new MSC/VLR (not shown) in the PLMN 105. The new MSC/VLR informs the HLR 115 of the new location of the terminal 130, and the HLR 115 sends a cancel-location message to the previously visited MSC/VLR 120 of the VPLMN 103 to indicate that the terminal 130 is no longer present in the area served by VPLMN 103. The HLR 115 sends an insert-subscriber-data (ISD) message to the new MSC/VLR of the VPLMN 105, providing the new MSC/VLR with the relevant subscriber data.
This architecture supports simultaneous activities in different MSC/VLRs in case of subscriber movement. In addition, activities on different gateway mobile switching centers (GMSCs) may be maintained simultaneously in various service scenarios. Typically, each PLNM has a GMSC, as indicated by GMSCs 140, 145 in FIG. 1. One scenario in which there are simultaneous subscriber activities is optimal routing, a network feature that enables calls to or attempts to initiate communication sessions with a subscriber to be routed directly to the subscriber's actual location or to the subscriber's forwarded-to destination. Without optimal routing, calls are routed via the HPLMN or, in the case of late call forwarding, via the VPLMN. In an exemplary optimal routing scenario, an HLR may receive send-routing-information (SRI) messages from other GMSCs that do not belong to the HPLMN network operator. An interrogating PLMN, which is the PLMN associated with an interrogating GMSC in an optimal routing scenario, interrogates the BPLMN of the called subscriber to determine the subscriber's location. The interested reader may refer to the GSM Technical Specifications (GTS) 02.79 and 03.79 for more details of the optimal routing feature.
Digital cellular communication systems such as those in accordance with the GSM standard have expanded functionality for optimizing system capacity and supporting hierarchical cell structures, i.e., structures of macrocells, microcells, picocells, etc. The term "macrocell" generally refers to a cell having a size comparable to the sizes of cells in a conventional cellular telephone system (e.g., a radius of at least about 1 kilometer), and the terms "microcell" and "picocell" generally refer to progressively smaller cells. For example, a microcell might cover a public indoor or outdoor area, e.g., a convention center or a busy street, and a picocell might cover an office corridor or a floor of a high-rise building. From a radio coverage perspective, macrocells, microcells, and picocells may be distinct from one another or may overlap one another to handle different traffic patterns or radio environments.
FIG. 2 illustrates an exemplary hierarchical, or multi-layered, cellular communication system that may be included in a PLMN. An umbrella macrocell 10 represented by a hexagonal shape makes up an overlying cellular structure. Each umbrella cell may contain an underlying microcell structure. The umbrella cell 10 includes microcell 20 represented by the area enclosed within the dotted line and microcell 30 represented by the area enclosed within the dashed line corresponding to areas along city streets, and picocells 40, 50, and 60, which cover individual floors of a building. The intersection of the two city streets covered by the microcells 20 and 30 may be an area of dense traffic concentration, and thus might represent a hot spot.
FIG. 3 is a block diagram of the exemplary cellular communication system, including an exemplary mobile station (MS) 130 and base station (BS) 135. The BS 135 includes a control and processing unit 150 which is connected through a base station controller (not shown) to an MSC/VLR 120 which in turn is connected to the public switched telephone network (PSTN) 155 (shown in FIG. 1). General aspects of such cellular radiotelephone systems are known in the art. The BS 135 handles a plurality of voice and data channels through a channel transceiver 160, which is controlled by the control and processing unit 150. Also, each BS includes a control channel transceiver 165, which may be capable of handling more than one control channel. The control channel transceiver 165 is controlled by the control and processing unit 150. The control channel transceiver 165 broadcasts control information over a control channel of the BS or cell to MSs locked to that control channel. It will be understood that the transceivers 160, 165 can be implemented as a single device, like the voice and control transceiver 170, for use with control and traffic channels that share the same radio carrier.
The MS 130 receives the information broadcast on a control channel at its voice and control channel transceiver 170. Then, the processing unit 180 evaluates the received control channel information, which includes the characteristics of cells that are candidates for the MS to lock on to, and determines on which cell the MS should lock. Advantageously, the received control channel information not only includes absolute information concerning the cell with which it is associated, but also contains relative information concerning other cells proximate to the cell with which the control channel is associated, as described for example in U.S. Pat. No. 5,353,332 to Raith et al., entitled "Method and Apparatus for Communication Control in a Radiotelephone System".
In North America, a digital cellular radiotelephone system using time division multiple access (TDMA) is called the digital advanced mobile phone service (D-AMPS), some of the characteristics of which are specified in the TIA/EIA/IS-136 standard published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA). Another digital communication system using direct sequence code division multiple access (DS-CDMA) is specified by the TIA/EIA/IS-95 standard, and a frequency hopping CDMA communication system is specified by the EIA SP 3389 standard (PCS 1900). The PCS 1900 standard is an implementation of the GSM system.
Several proposals for the next generation of digital cellular communication systems are currently under discussion in various standards setting organizations, which include the International Telecommunications Union (ITU), the European Telecommunications Standards Institute (ETSI), the Telecommunications Industry Association (TIA), and Japan's Association of Radio Industries and Businesses (ARIB) and Telecommunications Technology Council (TTC). Besides transmitting voice information, the next generation systems can be expected to carry packet data and to inter-operate with packet data networks that are also usually designed and based on industry-wide data standards such as the open system interface (OSI) model or the transmission control protocol/Internet protocol (TCP/IP) stack. These standards have been developed, whether formally or de facto, for many years, and the applications that use these protocols are readily available. The main objective of standards-based networks is to achieve interconnectivity with other networks (e.g., circuit-switched and packet-switched networks). The Internet is today's most obvious example of such a standards-based packet data network in pursuit of this goal.
In most of these digital communication systems, communication channels are implemented by frequency modulating radio carrier signals, which have frequencies near 800 megahertz (MHz), 900 MHz, 1800 MHz, and 1900 MHz. In TDMA systems and even to varying extents in CDMA systems, each radio channel is divided into a series of time slots, each of which contains a block of information from a user. The time slots are grouped into successive frames that each have a predetermined duration, and successive frames may be grouped into a succession of what are usually called superframes. The kind of access technique (e.g., TDMA or CDMA) used by a communication system affects how user information is represented in the slots and frames, but current access techniques all use a slot/frame structure.
Time slots assigned to the same user, which may not be consecutive time slots on the radio carrier, may be considered a logical channel assigned to the user. During each time slot, a predetermined number of digital bits are transmitted according to the particular access technique (e.g., CDMA) used by the system. In addition to logical channels for voice or data traffic, cellular radio communication systems also provide logical channels for control messages, such as paging/access channels for call-setup messages exchanged by BSs and MSs and synchronization channels for broadcast messages used by MSs and other remote terminals for synchronizing their transceivers to the frame/slot/bit structures of the BSs. In general, the transmission bit rates of these different channels need not coincide and the lengths of the slots in the different channels need not be uniform. Moreover, next generation cellular communication systems being considered in Europe and Japan are asynchronous, meaning that the structure of one BS is not temporally related to the structure of another BS and that an MS does not know any of the structures in advance.
In the ongoing standardization of next generation systems, ETSI and ARIB/TTC are cooperating to achieve a single standard common for both regions, the universal mobile telephone system (UMTS). The UMTS is based on a new radio access interface and an evolved version of a GSM core network. In addition to co-operating with ETSI, ARIB/TTC are cooperating with TIA to achieve a single standard common for both regions, the International Mobile Telecommunication 2000 (IMT-2000) standard. The IMT-2000 standard is based on an evolved CDMAone (i.e., TIA/EIA/IS-95-A) radio access interface (CDMA2000) and an evolved version of an ANSI IS-41 core network.
One feature of UMTS/IMT-2000 that may be implemented is a page-before-routing, or pre-page, feature. The idea is that the MSC/VLR should initiate the paging of a subscriber terminal 150 when the MSC/VLR receives a request for a roaming number (e.g., an MSRN) from the HLR and that the roaming number should not be not sent to the HLR until a page response is received from the subscriber terminal, indicating that the terminal is actually reachable at the moment. In the current procedure in GSM, the MSC/VLR returns a roaming number promptly upon request by the HLR and paging is not initiated until the call is actually routed to the MSC/VLR based on the roaming number.
The pre-page procedure has at least one advantage. If the subscriber terminal does not respond to the page so the call cannot be completed (which can happen quite often), then no switching and transmission resources between the GMSC and the MSC/VLR are (uselessly) reserved, as would be the case with the conventional GSM procedure. Hence, the pre-page feature saves switching and transmission resources in the fixed network. It is currently believed that this is the main reason for the feature.
If the pre-page feature is simply an option in the evolved GSM core network, some PLMNs can be expected to implement it and others can be expected not to implement it. This introduces a timer problem in the GMSC and the HLR. Paging a subscriber terminal takes much longer time than just allocating a roaming number, maybe a couple of hundred times longer. As a result, the HLR will not know in advance how long to wait for a response to its request for a roaming number so the IHLR will not know how to set its response-monitoring timer appropriately. If the conventional-GSM short timer is used with an MSC/VLR using the pre-page feature, the HLR's timer will time out and the call may be incorrectly released. On the other hand, if a long timer is used with an MSC/VLR not using the pre-page feature, the HLR's timer will be far too long and call completion will be unnecessarily delayed. The same considerations apply to the GMSC in relation to its communication with the HLR. Therefore, the use of the pre-page feature has to be coordinated somehow between the involved 5 nodes/networks.